Insectlike Flapping Wings in the Hover Part I: Effect of Wing Kinematics

Abstract

A GILE flight inside buildings, caves, and tunnels is of significant military and civilian value and is an attractive application for micro air vehicles (MAVs), defined here as flying machines of the order of 150 mm in size. Indoor flight imposes particular design and performance requirements, including small size, low speed, hovering capability, high maneuverability at low speeds, and (for covert operations) small acoustic signature, among other things. As discussed elsewhere [1–4], insectlike flapping is a solution thatmeets these requirements and is proven in nature. Although a number of elements characterize the design of a flapping-wing MAV, the focus here is on its wing aerodynamic design. This is crucial because for a flapping-wing MAV (FMAV) the wings are not only responsible for lift, but also for propulsion and maneuvers. Although insect flapping wings offer a proven solution and are abundant in nature (there are over 170,000 species of flying insects), little is known about the optimality of their wing design. Unlike forfixed or rotarywings, the parametric space associatedwith flapping wings is largely unexplored. A study that addresses the effects of both wing kinematics and wing geometry on the aerodynamic performance of flapping wings is required, and the former forms the underlying theme of this paper. The effect of wing geometry is considered elsewhere [5]. This work also provides insights into flapping-wing flow physics and uses these insights for aerodynamic design. Although Ellington’s [6] seminal work rejuvenated interest in insect flight, it is only recently that attention has been directed toward the design of vehicles that use insectlike flapping wings, particularly at the MAV scale [1–3]. In a later study, Ellington [7] proposed design guidelines based on scaling fromnature, but this does not give physical insight or allow design optimization. Dickinson et al. [8] investigated the effect of advancing or delaying pitch rotation of the flapping wing with respect to its translational motion, using experiments on Dickinson’s Robofly: a scaled-up mechanical model of the fruit fly Drosophila. Ramamurti and Sandberg [9] used a computational fluid dynamics (CFD) method to demonstrate this effect and presented some useful flow visualization. Sun and Tang [10] also used a CFD code to investigate the effect of advancing and delaying pitch rotation on insectlike flapping flight and the effect of varying the duration of stroke reversals [11]. In an earlier study [12], they investigated the effect of Reynolds number and the duration of wing stroke reversal. They also studied the effect of advance ratio (the ratio of flight speed to wing mean tip speed) in forward flapping flight [13]. Yu and Tong [14] used an aerodynamic modeling approach [15] to study forward flapping flight at various advance ratios by varying asymmetries between upand downstrokes. However, none of the preceding studies aimed to produce an optimized wing aerodynamic design. Milano and Gharib [16] made probably the only study thus far aimed at optimizing wing kinematics. They used a genetic algorithm paired with digital particle-image velocimetry experiments on a flapping wing in a water-filled towing tank. By using insectlike kinematics, they optimized for average lift over four flapping cycles and found a number of convergent solutions in the parameter space. They noted that the optimally efficient solutions all tended to generate leading-edge vortices ofmaximum strength. However, their Received 26 October 2007; revision received 11 June 2008; accepted for publication 13 June 2008. Copyright © 2008 by SalmanA. Ansari. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/08 $10.00 in correspondence with the CCC. ResearchOfficer, Department ofAerospace, Power and Sensors.Member AIAA. Professor of Aeromechanical Systems, Department of Aerospace, Power and Sensors. Associate Fellow AIAA. Reader in Control Engineering, Department of Aerospace, Power and Sensors. Member AIAA. JOURNAL OF AIRCRAFT Vol. 45, No. 6, November–December 2008